This invention relates to an improvement in the flame resistance of polyesters, and more particularly to novel flame resistant polyesters prepared by adding a specific phosphorus compound having at least two ester-forming functional groups during the preparation of aromatic polyesters.
In recent years, from the fireproof viewpoint it is increasingly required to provide various synthetic fibers and plastic products with flame resistance. An aromatic polyester such as polyethyleneterephthalate is one of the polymers which is now largely used as molded articles such as fiber, film, and board. Accordingly it has remained desirable to develop an aromatic polyester having flame resistance.
Conventional methods for providing a molded article consisting of aromatic polyester with the flame resistance are listed below.
(1) APPLYING A FLAME RESISTANT COATING TO THE SURFACE OF THE MOLDED ARTICLE.
(2) COMPOUNDING AND KNEADING A FLAME RESISTANT AGENT DURING THE MOLDING OF THE ARTICLE.
(3) MIXING A FLAME RESISTANT AGENT DURING THE PREPARATION OF POLYESTER.
(4) COPOLYMERIZING POLYESTER-FORMING REAGENTS WITH A FLAME RESISTANT AGENT DURING THE PREPARATION OF POLYESTER.
Among these methods the last copolymerizing method is most preferred in principle, because a molded article of polyester copolymerized with a flame resistant agent has the following superior characteristics in comparison with those obtained by the other preceding three methods; complicated operations after molding are obviated, the light resistance and the hand are not adversely affected, the flame resistance is maintained substantially uniform for long periods of time, the toxicity in use is negligible, and so on. For such a copolymerization process a number of phosphorus compounds as the flame resistant agent have hitherto been proposed.
These proposed phosphorus compounds, however, have serious disadvantages; the phosphorus compounds are expensive and thus inconvenient in practice since it is difficult to synthesize them because of their complicated chemical structures, they deactivate catalysts for polymerization to lower the rate of polymerization when added in polymerization systems for polyester, they give rise to side reactions such as formation of ether bonds and gelation, resulting in undesirable polymers having unsatisfactorily low physical properties, and they tend to dissipate from the polymerization systems in the normal polymerization conditions. As the result of this, on the one hand, they are required in greater amounts in order to obtain desirable polymers having the satisfactory flame resistance and, on the other hand, there occurs a pollution problem caused by their dissipation. Though it is intensely desired to manufacture flame resistant polyesters through the copolymerization with phosphorus compounds, a number of problems as described above have prevented the achievement of such a process.